1. Field of the Invention
The present invention relates to a conveying apparatus that attracts a recording medium to a conveyance carrier (e.g., conveyance belt) and conveys the recording medium, and a recording apparatus that performs a recording operation on the recording medium conveyed by the conveying apparatus.
2. Description of the Related Art
Inkjet recording apparatuses that eject ink from a recording head to a recording medium which is conveyed by a conveyance unit are known. Inkjet recording apparatuses can have compact recording heads, record high-definition images with high speed, have cheap running cost, and generate less noise because the apparatus employs a non-impact method. Further, inkjet recording apparatuses can record color images using multi-color ink. Particularly, a full-line type apparatus that employs a line-type recording head in which a large number of discharge ports are arranged in a width direction of the recording medium can perform a recording operation with high speed.
However, when the full-line type apparatus has a great number of line-type recording heads in a conveyance direction of the recording medium, a distance between the recording heat at the most upstream side and the recording head at the most downstream side is long. Therefore, if water content in the recording medium increases in the recording area, the recording medium may be lifted. As a result, the ink discharged from the recording head cannot impact a desired position of the recording medium affecting recording quality. To prevent the influence described above, the recording medium has to be urged toward the conveyance unit so that the recording medium is not lifted.
A method is generally known, which urges the recording medium to the conveyance unit in which an electrode is provided to the conveyance unit, and charges are applied to the electrode to generate electrostatic force to attract the recording medium to the conveyance unit (e.g., Japanese Patent Application Laid-Open No. 2002-284383).
To improve recording quality, a distance between an image recording unit for forming an image on a recording medium and a recording-medium conveyance carrier is set to be 1.0 mm or less in order that a flying speed and flying direction of a main ink droplet which contributes to the recording are not affected by external factors. This configuration aims to improve precision of the ink impact position.
However, when the recording medium is fed to the conveyance carrier, and a voltage applying unit applies voltage to an electrode of an electrode layer of the conveyance carrier to generate electrostatic attraction force, polarization is caused in the recording medium, so that voltage of several hundred volts (V) is induced on a front surface (recording surface) of the recording medium.
When an amount of a discharged main ink droplet is several ten picoliters (pico means parts per trillion), the voltage of several hundred volts (V) dose not give much influence on the ink discharge. However, when the amount of the main ink droplets is decreased to as small as several picoliters to improve the recording quality, the flying speed and the flying direction are apt to be affected by a surface potential of the recording medium, so that a stable recorded image cannot be obtained. Further, small mist-like droplets may be produced in addition to the main ink droplets that contribute to the recording. When the amount of the ink droplets is decreased to several picoliters, the production of mist-like droplets increases. Further, the generated mist is charged due to the voltage induced on the front surface of the recording medium, moves in a direction opposite to the recording direction, and impacts on a surface of a nozzle of the recording head, resulting in defective discharge.
It has been demanded in recent years that not only one surface of a recording medium, but both surfaces thereof, be subject to the recording operation.
When the recording operation is performed on both surfaces of the recording medium, ink is discharged to a first surface by a recording head, and then, ink is discharged to a second surface from the recording head. In this case, when the recording operation is performed on the first surface, a water content in the recording medium increases, so that the recording medium may greatly swell compared to a case where the recording operation is performed on only one surface thereof. Therefore, the recording medium may cause wrinkle (i.e., cockling). This phenomenon may affect the recording quality.
When a two-sided conveyance is employed using the conventional technique described above recording quality may suffer.
If the charged amount (applied voltage) for attracting the recording medium is decreased, the charge amount polarized on the surface of the recording medium is also decreased, so that image disturbance is not caused. However, there is a need to stably transport various types of recording media, including a recording medium that is likely to be curled and a recording medium having high rigidity. Therefore, to surely attract the surface of various types of recording media, to address different smoothness on the recording medium, by means of electrostatic force, the charged amount (applied voltage) has to be increased. In particular, during the two-sided printing operation described above, a surface resistance value of the recording medium is reduced due to the water content in the ink when the first surface is printed. Alternatively, the recording medium is swelled to extremely deteriorate the smoothness, so that a contact area between the conveyance carrier and the recording medium is reduced (the recording medium is difficult to be attracted).
The recording medium is susceptible to the humidity of the environment in which the recording medium is placed. Therefore, depending on the humidity, the surface potential of the recording medium will change making attraction by the recording medium difficult.